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The Influence of Salinity on the Diet of Nesting Bald Eagles

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The Influence of Salinity on the Diet of Nesting Bald Eagles J. Raptor Res. 42(2):99–109 E 2008 The Raptor Research Foundation, Inc. THE INFLUENCE OF SALINITY ON THE DIET OF NESTING BALD EAGLES A. CATHERINE MARKHAM1 AND BRYAN D. WATTS Center for Conservation Biology, College of William and Mary, P.O. Box 8795, Williamsburg, VA 23187 U.S.A. ABSTRACT.—Although the breeding density of Bald Eagles (Haliaeetus leucocephalus) in the lower Chesapeake Bay is known to vary with salinity, the ecological factors that contribute to this distribution have not been explored. In an effort to examine whether variation in prey use is associated with nest density patterns, we investigated the influence of salinity (tidal-fresh vs. mesohaline zones) on Bald Eagle diet composition by using video-monitoring to observe food delivered to nests during the 2002–2003 breeding seasons. Deliv- ered prey items were identified to the lowest taxonomic level possible and sizes were estimated relative to eagle bill length. We used species-specific length-weight relationships for prey to estimate biomass delivery. Overall, the diet included at least 12 species of fishes, three species of birds, four species of mammals, and four species of reptiles. Salinity had no significant influence on diet composition; Ictaluridae and Clupei- dae species were the most frequent prey items in both salinity zones. We suggest that pairs nesting in both tidal-fresh and mesohaline zones have access to similar fish species. However, the length and biomass of fish prey varied with salinity such that larger prey on average were delivered to nests in the mesohaline reaches compared to tidal-fresh zones. Thus, we suggest that foraging eagles may be exploiting more energetically- favorable conditions in higher salinity waters. Temporally, diet composition varied between study years, potentially reflecting annual changes in the availability of prey species. We consider differences in weather patterns between study years as the most likely factor contributing to this interannual variation in diet. KEY WORDS: Bald Eagle; Haliaeetus leucocephalus; anadromous; Chesapeake Bay; diet; salinity. INFLUENCIA DE LA SALINIDAD SOBRE LA DIETA DE HALIAEETUS LEUCOCEPHALUS DURANTE LA ANIDACIO´ N RESUMEN.—Aunque se sabe que la densidad de aves reproductivas de la especie Haliaeetus leucocephalus en la parte baja de la bahı´a de Chesapeake varı´a con la salinidad, los factores ecolo´gicos que contribuyen a esta distribucio´n no han sido explorados. En un esfuerzo para examinar si la variacio´n en el uso de presas esta´ asociada con los patrones de densidad de nidos, investigamos la influencia de la salinidad (zona mareal- dulce vs. zona mesohalina) sobre la composicio´n de la dieta de H. leucocephalus. Para ello utilizamos ca´maras de video para monitorear el alimento llevado a los nidos durante las e´pocas reproductivas de 2002 y 2003. Los´tems ı de presas llevados fueron identificados al menor nivel taxono´mico posible y sus taman˜os fueron estimados con relacio´n a la longitud del pico de las a´guilas. Utilizamos relaciones de longitud-peso especı´ficas para las especies de presa para estimar la biomasa de alimentos entregada. En general, la dieta incluyo´ al menos 12 especies de peces, tres de aves, cuatro de mamı´feros y cuatro de reptiles. La salinidad no tuvo una influencia significativa sobre la composicio´n de la dieta; las especies de Ictaluridae y Clupeidae fueron los´tems ı ma´s frecuentes entre las presas en ambas zonas de salinidad. Sugerimos que las parejas que anidan tanto en las zonas mareales-dulces como en la mesohalinas tienen acceso a especies de peces similares. Sin embargo, la longitud y la biomasa de peces presa vario´ con la salinidad de modo que, en promedio, se entregaron presas ma´s grandes en los nidos de las a´reas meso- halinas que en los de las a´reas mareales-dulces. Por lo tanto, sugerimos que las a´guilas podrı´an estar explotando condiciones energe´ticas ma´s favorables al buscar alimento en aguas con mayor salinidad. Temporalmente, la composicio´n de la dieta vario´ entre los an˜os de estudio, lo que potencialmente refleja cambios anuales en la disponibilidad de especies de presas. Consideramos que las diferencias en los patrones clima´ticos entre los an˜os de estudio son el factor que ma´s probablemente contribuye a la variacio´n interanual en la dieta. [Traduccio´n del equipo editorial] 1 Present address: Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Washington Rd., Princeton, NJ 08544 U.S.A.; Email address: [email protected] 99 100 MARKHAM AND WATTS VOL. 42, NO.2 Prey availability is a key determinant in the distri- METHODS bution (Dzus and Gerrard 1993), density (Gerrard We monitored 18 Bald Eagle nests in the lower et al. 1983), and success (Hansen 1987, Dykstra et al. Chesapeake Bay #3 km inland from the shorelines 1998, Gill and Elliot 2003) of breeding Bald Eagles of the James, York, and Rappahannock rivers during (Haliaeetus leucocephalus). However, directly assessing the 2002 (N 5 8) and 2003 (N 5 10) breeding prey availability in natural systems is difficult due to seasons (Fig. 1). Three areas were recognized along the broad geographic range and flexible foraging the estuarine salinity gradients of these tributaries: habits of Bald Eagles (Gende et al. 1997). As a result, tidal-fresh (0.0–0.5 ppt salinity), oligohaline (0.5– few investigations have examined variation in prey 5.0 ppt), and mesohaline (5.0–18.0 ppt; Chesapeake use within a breeding population and little has been Bay Program Monitoring Subcommittee Data Analy- reported about how diet varies along environmental sis Work Group unpubl. data). We limited nest gradients that determine prey distribution (but see selection to tidal-fresh and mesohaline reaches to Watson et al. 1991, Dzus and Gerrard 1993, Jackman (1) examine extremes in salinity effects within river et al. 2007). systems and (2) because Watts et al. (2006) docu- In a recent investigation of population parame- mented significant differences in breeding density ters for Bald Eagles in the lower Chesapeake Bay, between these salinity zones. Watts et al. (2006) found that shoreline areas sur- To place video-recording equipment, we selected rounding low saline waters support higher breeding nests that had a history of reliable production and densities, have higher reproductive rates, collective- regularly experienced human interaction (with the ly produce a greater number of young, and have reasoning that breeding pairs habituated to human experienced faster rates of population recovery presence were less likely to be disrupted by research- compared to shoreline areas surrounding higher er activity). Using these criteria, we selected nine saline waters. This implies tidal-fresh reaches repre- nests in each salinity zone. One nest in the mesoha- sent core breeding areas for Bald Eagles, yet the line reach was used in both years of this study; be- ecological advantages of these regions are un- cause nests were considered individual samples in known. One possible explanation for the observed all analyses, samples sizes presented reflect the pattern is that the influence of salinity on eagle unique pairing of nest location and year. breeding density is mediated through prey availabil- Data collection at each nest was determined by ity (Watts et al. 2006). In the Chesapeake Bay, Bald nestling age, as estimated during aerial surveys Eagles forage primarily on fish during their breed- and later confirmed by visual inspection of chicks. ing season (Wallin 1982, Mersmann 1989). Fish We divided the nesting cycle into three phases rel- communities are not uniform throughout the estu- ative to the expected period of maximum growth in arine ecosystem and salinity is one of the key factors developing eaglets: before (0–14 d), during (15– known to influence the abundance and distribution 45 d), and after (46 d–fledging) anticipated maxi- of fish species in Chesapeake Bay waters (Murdy et mum growth (Bortolotti 1984). Recording effort at al. 1997, Jung 2002). This, along with data indicat- all nests was focused primarily on the maximum ing that Bald Eagle breeding pairs typically forage growth phase. By this age, eaglets are endothermic within home ranges close to their nest site (,3 km; (14.7 d; Bortolotti 1984) and nest trees can be Buehler et al. 1991), suggests that eagles nesting in climbed for camera installation with minimal risk different salinity zones encounter different suites of associated with exposing chicks to ambient temper- prey species or experience different food resource atures. Further, nestlings in this phase experience levels. the fastest rate of growth (Ricklefs 1967) and, ac- Here, we investigate the influence of salinity on cordingly, provisioning rates have the greatest im- the diet composition of Bald Eagles in the lower pact on overall growth patterns (Bortolotti 1989). Chesapeake Bay to examine how breeding pairs re- Monitoring for two nests began at hatching, and spond to potential differences in prey communities thus, also included the pre-maximum growth phase and availability between salinity zones. Specifically, (cameras were installed prior to egg-laying); moni- we (1) describe the diet of breeding Bald Eagles in toring of 10 nests continued through fledging and the Chesapeake Bay region, (2) evaluate the extent therefore included the post-maximum phase. For to which salinity contributes to spatial variation in nests with multiple young, we used the hatch date diet, and (3) investigate patterns of variation in diet of the oldest nestling when assigning brood ages for between the two years of this study. data analysis. Hatch date was either (1) determined JUNE 2008 BALD EAGLE DIET 101 Figure 1. Locations of nests used (2002–2003) in the lower Chesapeake Bay study area. Nests are distinguished between those in tidal-fresh (N) and mesohaline (&) salinity zones.
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